The invention is directed to a chip component adapted for fastening to a circuit board comprising an electrical or electronic function member that is provided with coatings on at least two outside surfaces, that has band-shaped terminal elements for connecting the coatings to contact locations of a printed circuit situated on the circuit board, and in which the function member is built into a prefabricated housing in cup form composed of insulating material from which the terminal elements are conducted to the exterior to form contact surfaces.
A component comprising these features is shown in FIG. 7 and described in the appertaining text of DE-AS 10 64 127.
Following upon the efforts for miniaturizing electronic circuits, a new, space-saving technology also prevailed in the housing forms of discrete components. The currently smallest housing form is that of the surface-mounted devices (SMD) that are also referred to as chip components. Characteristic of these surface-multiple components is that both the member as well as the contact locations in them are situated on the interconnect side of circuit boards. In contrast to traditional, wired components whose terminal elements are conducted through holes in the circuit board onto the other side thereof in order to be soldered to the respective contact locations there, SMD components must themselves be resistant to soldering.
Since the SMD components are directly exposed to the soldering process, the materials employed must be designed for the wave soldering, immersion soldering or reflow soldering methods that are currently standard. The components are thereby exposed to a temperature of 260.degree. C. and more for about 10 seconds. Dependent on the specific electrical or electronic function members to be protected, SMD technology therefore requires that one or more additional protective measures be undertaken in order to guarantee the heat resistance of the component during the soldering process.
Such components together with protective measures therefor are known for components such as film capacitors.
German Published Application 34 12 492 (corresponding to European Patent A 0 162 149 or, respectively, U.S. Pat. No. 4,617,609), for example, discloses a chip component capacitor wherein the capacitor member is coated with a cuboid envelope and is protected by a heat trap. The heat conduction between the solder bath and the capacitor is diminished on the basis of a cross sectional variation of the terminal elements.
It is presently mainly the measures of tempering and the selection of special materials for the insulating housing or providing a great wall thicknesses thereof that are employed in order to be able to keep the changes in the electrical values of the capacitor member reliably within the prescribed limits as a consequence of the immersion soldering process for example, the relative change in capacitance must not exceed 2%. The enveloping of MK capacitors with high-temperature-proof thermoplastics that have good heat insulation such as PPS (polyphenylensulfide) is also disclosed, for example, in German Published Application 34 12 492.
The present invention can be employed for all chip components; however, it is preferably employed for those chip components whose function members are heat-sensitive. For example, capacitors having plastic as a dielectric that carries a thin metallization (MK capacitors), tantalum and ceramic capacitors, resistors, thermistors and posistors, inductances, relays, semiconductors, hybrid circuits, etc. are such chip components.
Up to now, the housings of the chip components are manufactured in that the function members are coated or, respectively, injection coated. This execution of the chip components is problematical for several reasons: first, this requires the acquisition of expensive extruding machines or, respectively, expensive transfer molding forms. Moreover, long curing times of the housing material and high dead head losses amounting to up to 50% of the material consumption must be accepted.
During the coating or injection molding process, second, the function members of the components have their electrical values changed to a greater or lesser degree due to the temperatures, pressures and mass stream velocities (abrasion) that thereby occur. This can lead to what is often a considerable reject rate.
Further, there is the risk that crack formations in the enveloping housing will appear during the immersion soldering process. The reason for this is that the function member exerts a mechanical load onto the walls of the enveloping housing during thermal stressing. The risk of crack formations can be avoided in that the wall thicknesses of the housing are increased by 50 through 80%, i.e. to 0.7 mm and more. In addition to an increase in the cost of materials, of course, this also involves an enlargement of the dimensions of the chips.
Finally, what is referred to as a deflashing is necessary after the coating or, respectively, injection molding process. Slight quantities of the injection molding compound are thereby removed, these having penetrated through seams to locations which contact the component and having adhered there.
For the specific case of metallized plastic capacitors, the application of this prior technique involves the special disadvantage that the component or function member must be tempered in a relatively involved procedure before the immersion soldering process in order to obtain an adequate resistance to temperature. The capacitance of the capacitors is reduced due to this manufacturing step and additional rejects can arise, this leading to increased manufacturing costs overall.